This invention relates to a process for improving the storage stability of lube oil base stocks derived from a neutral oil. By "storage stability" is meant the resistance of the oil to forming a floc in the presence of oxygen. The process comprises three steps. In the first step a neutral oil stock is catalytically dewaxed to produce a dewaxed stock. In the second step the dewaxed stock is hydrofinished to produce a partially hydrogenated dewaxed stock. In the third step the partially hydrogenated dewaxed stock is stabilized by nonhydrogenative acid stabilization. It has been found that the storage stability of the resultant lube oil base stock is significantly improved.
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, there are quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 550.degree. F., the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types. This complexity and its consequences are referred to in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference for background.
The basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant base stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant base stock currently consists of a set of unit operations to remove or convert the unwanted components. The most common of these unit operations include, for instance, hydrocracking, dewaxing, distillation, and hydrofinishing.
The lubricant base stock isolated by these refining operations may be used as such as a lubricant, or it may be blended with another lubricant base stock having somewhat different properties. Or, the base stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and viscosity index improvers. As used herein, the term "stock", regardless whether or not the term is further qualified, refers to a hydrocarbon oil without additives. The term "raw stock" refers to an untreated viscous distillate or the residuum fraction of crude petroleum oil isolated by vacuum distillation of a reduced crude from atmospheric distillation, or its equivalent. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "base stock" will refer to an oil refined to a point suitable for some particular end use, such as for preparing automotive oils.
In general, refineries do not manufacture a single lube base stock but rather process at least one distillate fraction and the vacuum residuum. For example, three distillate fractions differing in boiling range and the residuum may be refined. These four fractions have acquired various names in the refining art, the most volatile distillate fraction often being referred to as the "light neutral" fraction or oil. the other distillates are called "medium neutral" and "heavy neutral" oils. The vacuum residuum, after deasphalting, solvent extraction and dewaxing, is commonly referred to as "bright stock". Thus, the manufacture of lubricant base stocks involves a process for producing a slate of base stocks, which slate includes at least one refined distillate.
Processes have been proposed to produce lubricating oils from ordinary and low quality base stocks derived from neutral oils which normally would be converted into other products. Many of these processes use a hydrocracking step. The problem is that such hydrocracked stocks tend to have poor storage stability.
Some process schemes proposed for hydrocracked stocks also involve the use of catalytic dewaxing to lower the pour point of the final lubricant base stock. The product of catalytically dewaxing a hydrocrackate stock tends to be unstable such that further processing is required to make a stable oil.
The typical dewaxed hydrocrackate stock is hydrofinished by a mild hydrogenation process to increase the resistance of the bulk oil toward oxidation. The goal of this process is to hydrogenate those species which readily react with oxygen, while minimizing further cracking and loss of the lubricant base stock. Even though the hydrofinished product has high resistance toward bulk oxidation, its storage stability is often low. It is believed that this is due to the difficulty of totally saturating the floc-forming agents, thought to be partially hydrogenated polycyclic aromatics. These agents, upon reaction with oxygen, can lead to floc formation during storage of the oil.
There are several nonhydrogenation processing techniques recommended in the patent literature as methods to achieve improved lubricant storage stability. Some of the earlier efforts concentrated on the addition of stabilizing agents to a dewaxed hydrocrackate while in the presence of a heterogeneous acidic catalyst. Several issued patents relate to stabilizing hydrocracked lubricant base stocks by adding stabilizing agents such as olefins, alcohols, esters or alkylhalides to the lube stock while in the presence of a heterogeneous acidic catalyst such as acid resins, clays, and aluminosilicates having controlled alkylation activity. For instance, U.S. Pat. No. 3,928,171, Yan et al., Dec. 23, 1975 and U.S. Pat. No. 4,181,597, Yan et al., Jan. 1, 1980 disclose processes for stabilizing hydrocracked lube oils which have been dewaxed, preferably solvent dewaxed, by contacting them with stabilizing agents such as C.sub.6 to C.sub.10 olefins. The stabilizing catalyst is a heterogeneous acidic catalyst; the olefins which are not consumed in the reaction are distilled off from the product and combined with other olefins to be added into the stabilizing reaction zone feed. As discussed in U.S. Pat. No. 4,181,597 the mechanism responsible for the benefits obtained when using a stabilizing agent was not entirely understood. However, since the stabilizing agent is consumed during the stabilization reaction, it is likely that a reaction occurs between one or more components of the dewaxed lube oil stock and the stabilizing agent. In particular, conditions during the stabilization process are conducive to alkylation. Nonetheless these earlier efforts refrain from asserting that any particular mechanism can be identified as the stabilizing reaction.
Subsequent to the disclosure of the use of a stabilizing agent it was found that if the dewaxing step was carried out using as a catalyst a low aluminum content intermediate pore size zeolite substantially free of hydrogen transfer activity, the resulting dewaxed effluent comprising in situ generated olefins when contacted with an acidic alkylation catalyst produced a low pour point, high stability, low color, lube stock in good yields. U.S. Pat. No. 4,361,477, Nov. 30, 1982, describes a two-step process for dewaxing and stabilizing hydrocracked stocks. The dewaxing step uses a high-silica zeolite catalyst to produce catalytic dewaxer effluent containing lower olefins for favorable stabilizing (alkylation) reactions. The stabilizing step uses an acidic catalyst substantially free of hydrogenation activity to stabilize the dewaxed lube oil. The primary advantage of the two-step process resides in the improved stability of the lube oil product obtained in good yields, without the need to use a stabilizing agent. The mechanism responsible for the stabilizing reaction again appears to be an alkylation reaction between the unstable polycyclic aromatics present in the dewaxer effluent and the olefins also present. This alkylation process has been called nonhydrogenative stabilization.
Thus, at the time of the present invention, the patent literature relating to lube oil stabilization taught nonhydrogenative stabilization (alkylation) as an alternative to hydrofinishing a dewaxed stock. Furthermore, the patent literature taught two approaches to nonhydrogenative stabilization. In one approach, a conventionally dewaxed stock is stabilized when contacted with an externally supplied stabilizing agent in the presence of an acidic catalyst. In a second approach, a hydrocrackate stock is dewaxed under conditions selected to internally generate an olefinic stabilizing agent during dewaxing and then stabilized using an acidic catalyst and the internally supplied stabilizing agent. These two approaches, both apparently relying upon an alkylation reaction, have been employed as alternatives to conventional hydrofinishing, and those familiar with lube oil stabilization have identified the occasions when the advantages of each approach over the other can be better utilized.
In spite of the large amount of research into developing lubricant base stocks and stabilizing them, there continues to be intensive research into developing more efficient and more convenient method for achieving those goals. The object of the present invention is to provide such a more convenient and efficient process.
It has now been discovered that a three-step process comprising a catalytic dewaxing step and a two-step stabilization process in turn comprising a hydrofinishing step and a nonhydrogenative acid stabilization step using an added olefinic stabilizing agent results in improved stability. Thus, rather than employing the nonhydrogenative stabilization step as a substitute for hydrofinishing, the present invention employs it in addition to hydrofinishing when refining a dewaxed hydrocrackate neutral oil.